Known is a conventional spot welding head of the aforenoted type for spot-welding a steel strap longitudinally at one, two or three spots at regular intervals (Japanese Patent Publication H.2-274383).
There is another spot welding head for spot-welding a steel strap laterally at two or more spots at one or more predetermined intervals (Japanese Utility Model Publication H.7-31891).
Before these spot welding heads were developed, there were only available spot welding heads exclusively for spot welding at only one spot or longitudinally in only one line. With regard to those steel straps which were spot-welded at only one spot or longitudinally in only one line, however, if tensile force is applied to each of the straps from both sides of the weld spot, the welded minute area behaves in such a manner that it turns perpendicularly to the direction of tension. This causes a bending moment to act on the welded portion. As a result, relatively small forces broke some of such welded portions.
Therefore, spot welding heads for multi-point spot welding as mentioned above have been provided, and measures have been taken so that no bending moments act on the spot-welded portions, or so that the portions are sufficiently secure against bending moments, if any.
With regard to the above multi-point spot welding, if a tensile force is applied to a welded steel strap, a slight bending moment acts on the welded portions on both sides. In addition, because the strap is welded at multiple spots in a line within a size limited for compactness of the apparatus, the distances between the weld spots are shortened. Therefore, the spot welding temper hardening extremely reduces the allowable bending stress in the directions along the line connecting the weld spots, and also lowers the allowable stress in the direction of the tension. In the present situation, these reduced and lowered stresses are allowed within allowable ranges. In particular, for a steel strap of high-carbon material (with an equivalent carbon content of 0.2% or more), the above evils or ill effects of temper hardening due to spot welding are remarkable.
Therefore, a steel strap may be spot-welded in zigzag, square or rectangular modes. Because a spot welding head needs to be compact, however, it has been practically impossible to provide a spot welding head for automatic zigzag, square or rectangular spot welding.
In other words, it has been difficult to devise such a mechanism that can move a spot electrode in a zigzag, square or rectangular mode as stated above in a spot welding head, which has only a relatively narrow space.
Furthermore, the spot electrode of a conventional spot welding head for the foregoing multi-point spot welding is moved by a rotary actuator or the like through a gear mechanism. Therefore, the intervals between the nuggets of spot welding due to gear back lash are not accurate. For example, it was originally necessary for nuggets to have intervals of 10 mm, but their actual intervals were 8 mm and 9 mm. In addition, a spot welding head with this type of mechanism is complicated in structure and therefore expensive, and it is structurally impossible to easily adjust the spot welding intervals from the outside.